The present invention relates to a shaft for a golf club.
A golf club including a shaft made of a fiber-reinforced resin has recently been developed as disclosed in the Japan Utility Model Application No. 30359/78 etc. The outside diameter of the tip of the shaft is about 7 to 10 mm, or about 7 to 9 mm for a wood and about 8 to 10 mm for an iron. Since the thickness t of the main body 1 of the shaft is gradually deceased from the tip 3 of the body to the butt 5 thereof, the outside circumferential surface of the main body is tapered from the butt thereof to the tip thereof in order to increase the rigidity of the body toward the butt thereof, as shown in FIG. 22. The grip of the shaft is conjoined to the main body 1 at the butt 5 thereof. Since the tapering of the outside circumferential surface of the main body 1, which results in increasing the outside diameter thereof toward the butt thereof, is so large that the ratio of the decrease in the outside diameter to the length of the body is 6/1,000 or more, the resistance of air to the body at the time of swinging of the golf club is increased to slow down the swinging speed. This is a problem.
To solve the problem, a golf club shaft whose main body is tapered from the butt thereof to the tip thereof at a ratio of 0/1,000 to 4/1,000 so as to be nearly uniform in outside diameter has been proposed as disclosed in the Japan Utility Model Application No. 31743/85. Since the main body is nearly equal in the outside diameter of the tip to the main body of the former shaft, the latter shaft is higher in flexibility than the former shaft. Since the main body of the latter shaft is smaller in diameter as a whole than that of the former shaft, the resistance of air to the main body of the latter shaft is lower than that to the main body of the former shaft to make it possible to speed up the swinging of the golf club having the latter shaft. For that reason, even non-powerful golfers such as beginners and women can more quickly swing the golf club to move the head thereof faster to hit a ball to a larger distance. However, powerful golfers, who swing their golf clubs quickly, want such a golf club of lower air resistance more than the non-powerful golfers, and the magnitude of air resistance affects the powerful golfers more than the non-powerful golfers. Nonetheless, the golf club of lower air resistance is too high in flexibility or too low in rigidity for the powerful golfers. In other words, a golf club low enough in air resistance and high enough in rigidity for the powerful golfers has not existed.
Technic for winding a sheet material on a shaft main body to provide a reinforcing layer on the main body has been proposed.
The main body 17 of the shaft of a golf club disclosed in the Japan Utility Model Application (OPI) No. 131275/80 (the term "OPI" as used herein means an "unexamined published application") is manufactured by winding a plurality of fiber-reinforced resin sheets 11, 13 and 15 around a mandrel 27, and then winding fiber-reinforced resin sheets 19 and 21 around the former sheets so that the latter sheets constitute reinforcing layers 23 and 25, as shown in FIGS. 23 and 24, to partially heighten the rigidity of the main body to modulate the kick point of the shaft. However, since the reinforcing sheet 21 differs in strength from the former sheets 11, 13 and 15, the elongation of the reinforcing sheet and that of the former sheet 15, which are caused when the main body 17 flexes during the swinging of the golf club, differ from each other to make the sheets likely to separate from each other, as shown in FIG. 25. Besides, since the sheet-reinforced portion and sheet-unreinforced portion of the sheet 15 adjoin each other at the riser 29 of the reinforcing sheet 21, stress concentrates in the former sheet at the riser at the time of the flexing of the main body 17 to make the former sheet likely to undergo a crack 31 extending in the radial direction of the body, as shown in FIG. 26.
The main body of the shaft of a golf club disclosed in the Japan Utility Model Application No. 41728/91 is manufactured by winding a plurality of prepreg sheets each composed of high-strength fibers such as carbon fibers and fiberglass and a matrix of epoxy resin, polyester or the like, which is impregnated in the fibers. At the time of the manufacturing, each of the sheets is wound by one turn so as not to overlap the side edges of the sheet with each other but to butt the edges to each other, or each sheet 41 is wound by two turns so as to butt one side edge 41b of the sheet to a riser 45 formed by the sheet because of the overlapping of the other side edge 41a and other portion thereof, as shown in FIG. 27. The main body is slightly lower in strength at the buttings of the sheets than at the other portions thereof. As a result, if the buttings are located on an imaginary straight line extending along the axis of the main body, the rigidity thereof varies along the circumference thereof.
Since the main body of the shaft of a golf club is made of sheets wound so that the side edges of each of them are not butted to each other but overlapped with each other, the thickness of the layer constituted by the sheet is larger at the overlapping thereof than at the other portion thereof so that the rigidity of the main body varies along the circumference thereof.
For these reasons, the head of each of the last two golf clubs deflects at the time of swinging of the club so that a ball cannot precisely be hit therewith.